Modern wireless systems using wideband multiple access technologies such as wideband CDMA (code division multiple access), referred to as W-CDMA or CDMA2000, are expected to effectively handle large variety of services, such as voice and data services. Examples of data services include short message service (SMS), multimedia message service (MMS), web browsing, online gaming, etc. Often, these latter services may require that data be transmitted at a rate much higher than that required for voice transmission.
The link between a mobile user station (MU) and a base transceiver station (BTS) in a CDMA system is a multiple radio access channel, where users share the same physical radio channel (i.e. the signals transmitted by an MU occupy the same bandwidth as the signals transmitted by other MUs in the respective cell), each MU being uniquely identified by a spreading code. To support various end user services, an MU transmitter divides this physical channel in sub-channels. One sub-channel (the pilot sub-channel) is used to manage the air interface resources and enables the BTS to keep track of the mobile terminals currently under its control. The remaining sub-channels are used for traffic and traffic control. More particularly, a fundamental sub-channel (FCH) is used for voice traffic, a supplemental sub-channel (SCH) is used for data traffic, and a control channel (DCCH) for signalling and synchronization.
The requirements for voice and data transmissions are different in many respects: the voice signals must be processed substantially in real-time, while the data signals can tolerate some degree of delayed operation. To accommodate the bursty nature of the high-speed data services, it is necessary to assign a large data bandwidth to the respective call for the duration of the data burst.
Using the reverse link (from the MU to BTS) as an example, a mobile terminal starts transmitting when it has gathered enough data, and keeps idle when the transmission is over. When a data transmission is required, an MU is immediately assigned a SCH. It is useful to know when a burst transmission is over to optimize the bandwidth usage.
For data services, or any other services in which the transmissions are infrequent bursts, the base station may lose synchronization with its mobile terminals. Several methods have been proposed to avoid the loss of synchronization at the base transceiver station.
One method, known as a continuous mode of operation (CONT) scheme, proposes that the MU transmits continuously a low bit rate control signal to the BTS between data bursts, so that the base station could maintain synchronization with the terminal between data bursts. The CONT scheme requires reservation of an uplink channel decoding resources for each MU engaged in bursty data transmission; these resources cannot be used by any other mobile terminal and cannot be used when the assigned mobile terminal ceased transmitting. Additionally, the receiver on the base station must remain active at all times.
Another method, known as the discontinuous transmission medium access control (DTX) scheme, proposes to discontinue the transmission from the individual mobile terminals during the off-periods, and to share the freed BTS receiver decoding resources among different users. While the DTX scheme may achieve considerable savings in base station equipment, it requires transmitting a synchronization message at the beginning of each on-period to allow the base station to acquire synchronization with the transmitting mobile terminal. The synchronization message also serves to inform the base station that a particular mobile terminal intends to transmit.
Most CDMA systems are provided with both schemes. As a result, there is a need to provide the base station receiver with means to determine precisely whether the mobile operates in a continuous (CONT) transmission mode, or in a discontinuous transmission (DTX) mode. In addition, in a CONT scheme when the receiver decides that a received frame is in error, it erases the frame. The MU has to determine the number of erasure frames that were erased (erasure frames). Also, it is important for the receiver to determine if the transmitted frame has been erased due to certain conditions on the air link, or due to the transmission being discontinued. An incorrect determination of erasure frames will trigger unnecessary signalling (and occupy valuable bandwidth) between the MU and the BTS, and will also result in an unnecessary increase in the power level fluctuation.
A method to identify a DTX mode used currently is to decode a supplemental channel frame, and then encode a frame worth of the decoded bits, in the same manner as performed by the transmitter. The encoded symbols are then correlated with the decoder input. The correlation result is used as a metric and compared with a threshold to determine whether there is a frame transmitted, and whether the frame was received correctly. This method improves the DTX mode detection reliability for convolutional code systems, but not for turbo code systems, since the turbo decoder operates at a lower signal to noise ratio as compared to a Viterbi decoder, commonly used in decoding convolutional codes. In addition, use of the re-encoding and correlation introduce a considerable amount of computations.
Another method employed currently for identifying a DTX mode is to measure the transmitted and received power ratio of a pilot channel to the supplemental channel. A DTX mode is recognized if the power ratio is greater than a threshold. However, this method is not reliable for the convolutional code system or the turbo code system due to relatively low signal to noise ratio.
In CDMA2000 systems, voice is transmitted in the fundamental channel. To reduce interference and thus increase system capacity, variable data rate voice encoding technique is used. The voice encoder (vocoder) outputs the highest data rate (called full rate) when there is a full voice activity, and the lowest data rate (⅛th rate) when it detects silence. There are also other two intermediate rates, i.e., half rate and quarter rate for voice data rate transition. The voice data rate information is not explicitly sent. The base station and mobile receivers have to detect precisely the rate of the incoming voice frame. Incorrect rate determination results in annoying audible noise.